This invention relates to the field of controlling and reducing electric-field-induced damage during integrated circuit fabrication and, in particular, to protection against electric-field-induced damage in reticles.
Certain objects in semiconductor manufacturing, such as packaged devices, silicon wafers and reticles are prone to damage by field-induced potentials. A reticle is an optically clear ceramic substrate (typically quartz) having a thin metal (e.g., chromium) coating in which a pattern has been formed. Field-induced damage can occur even without any electrical contact being made with the sensitive object. The mechanism of damage has been widely identified as resulting from electrostatic discharge (ESD), a mechanism that occurs when the voltage between electrically isolated parts within a device or object is raised by field induction to a point at which a discharge occurs between them. There is a threshold for the occurrence of such discharge, below which the risk of damage is considered to be small. The various thresholds for discharge in different types of object have been estimated and “safe” levels for electric field around them have been prescribed in several places, such as the International Technology Roadmap for Semiconductors.
Much of the information available about static control was developed for spark avoidance where there is an explosion risk, such as in the handling of flammable liquids and vapors. Spark avoidance has also been a priority in semiconductor manufacturing since the radio emissions from spark events can cause data corruption in the electronics used to control process equipment.
To minimize the risk of ESD, it is common practice to control static charge levels in the environments where field-sensitive objects are handled. The guidance given by bodies such as the ESD Association of America for the control of ESD includes the avoidance of all insulators that are not necessary for the process, the use of static dissipative materials wherever the use of metal is inappropriate (e.g., for transparency or chemical inertness), and electrical grounding of all conductive or dissipative parts of the equipment through a common ground point. It is common practice to control static charge levels in handling environments. For example, grounded static dissipative materials are used in semiconductor manufacturing facilities (herein “fabs”) to minimize tribocharging of reticle pods and other handling equipment.
Electrostatic buildup on and discharge from reticles can damage or destroy the reticles, and concern about electrostatic damage has been increasing in recent years as device geometries get finer and the requirements for reliability become more stringent.
It is known to store and transfer workpieces, such as semiconductor wafers and reticles, using a standard mechanical interface, or SMIF, system. In conventional SMIF pods, it is known to have conductive contacts on the reticle support in the pod door to dissipate electrostatic charge from the bottom surface of the reticle. The charge is then grounded through the pod door. Similarly, conductive contacts are provided on the reticle retainer in the pod shell to dissipate electrostatic charge from the top surface of the reticle. The charge from the top surface is then grounded through the pod shell. The pod shell, therefore, typically includes static dissipative materials to provide a path to ground for the static charge from the top surface of the reticle. Another type of reticle container provides a conductive path between the reticle retainer and the reticle supports, as disclosed in U.S. Pat. No. 6,513,654, issued Feb. 4, 2003, to Smith et al. This allows electrostatic charge to be dissipated from the top surface of the reticle without the use of static dissipative materials in the pod shell.
Field-sensitive objects often carry electrical charge. An object can acquire a charge through tribocharging during handling, through normal processing such as rinsing with deionized water, through ionizer malfunction and by other events. Not all such situations can be avoided, particularly when charging is an unfortunate by-product of the process itself (such as washing). It is known to provide air ionizers to neutralize electric charges on objects.